Heretofore, as a positive active material for a nonaqueous electrolyte secondary battery which is represented by a lithium secondary battery, a “LiMeO2-type” active material (Me is a transition metal) having an α-NaFeO2-type crystal structure has been examined, and nonaqueous electrolyte secondary batteries including LiCoO2 have been widely put to practical use. However, the discharge capacity of LiCoO2 is about 120 to 130 mAh/g. As Me, it has been desired to use Mn that is abundant as an earth resource. However, a “LiMeO2-type” active material containing Mn as Me has the problem that when the molar ratio (Mn/Me) of Mn to Me is more than 0.5, the structure is changed to a spinel type-structure when the battery is charged, and thus it is unable to maintain a crystal structure, resulting in very poor charge-discharge cycle performance.
Thus, various kinds of “LiMeO2-type” active materials which have excellent charge-discharge cycle performance and in which the molar ratio (Mn/Me) of Mn to Me is 0.5 or less have been proposed and, and some of these active materials have been put into practical use. For example, a positive active material containing LiNi1/2Mn1/2O2 or LiNi1/3Co1/3Mn1/3O2 which is a lithium transition metal composite oxide has a discharge capacity of 150 to 180 mAh/g.
Meanwhile, in contrast with so called a “LiMeO2-type” active material as described above, so called a “lithium-excess-type” active material is also known in which the composition ratio Li/Me of lithium (Li) to the ratio of a transition metal (Me) is greater than 1, with Li/Me being, for example, 1.2 to 1.6.
Patent Document 1 discloses “a positive active material for a nonaqueous electrolyte secondary battery, the positive active material comprising a lithium transition metal composite oxide which has an α-NaFeO2 type crystal structure, and is represented by a composition formula Li1+αMe1−αO2 (Me is a transition metal element including Co, Ni and Mn, and α>0) and in which the molar ratio (Li/Me) of Li to the transition metal element Me is 1.2 to 1.6, wherein the molar ratio (Co/Me) of Co to the transition metal element Me is 0.02 to 0.23, the molar ratio Mn/Me of Mn to the transition metal element Me is 0.62 to 0.72, and the positive active material is observed as a single phase attributed to a space group R3-m on an X-ray diffraction diagram when electrochemically oxidized to a potential of 5.0 V (vs. Li/Li+)” (claim 1).
Patent Document 1 suggests in examples that a coprecipitation carbonate precursor of a transition metal element including Co, Ni and Mn and lithium carbonate are mixed, and fired to synthesize a lithium transition metal composite oxide (see paragraphs [0083] to [0086], [0109], Table 2 in paragraph [0119], and Table 3 in paragraph [0120]).
In addition, a positive active material for a nonaqueous electrolyte secondary battery is known which contains a lithium transition metal composite oxide in which the full width at half maximum for each of the diffraction peaks of the (003) plane and the (104) plane in X-ray diffraction measurement is defined (see, for example, Patent Documents 2 to 5).
Patent Document 2 discloses “a lithium secondary battery comprising: a current collector; and an active material layer held in the current collector and containing active material particles, wherein the active material particle is a secondary particle formed by aggregating a plurality of primary particles of a lithium transition metal oxide, and has a hollow structure with a hollow portion formed in the secondary particle and a shell portion surrounding the hollow portion, the secondary particle is provided with a through-hole extending through the secondary particle from the outside to the hollow portion, and in a powder X-ray diffraction pattern of the active material particle, the ratio (A/B) of a full width at half maximum A of a diffraction peak obtained from the (003) plane and a full width at half maximum B of a diffraction peak obtained from the (104) plane satisfies the following formula: (A/B)≤0.7” (claim 1), and “the secondary battery according to claim 1, wherein the lithium transition metal oxide is a compound having a layered crystal structure represented by the following general formula:Li1+xNiyCozMn(1−y−z)WαMβO2 (in the formula (1), x, y, z, α, and β are real numbers satisfying all of 0≤x≤0.2, 0.1<y<0.9, 0.1<z<0.4, 0.0005≤α≤0.01 and 0≤β≤0.01, and M is not present, or is one or more elements selected from the group consisting of Zr, Mg, Ca, Na, Fe, Cr, Zn, Si, Sn, Al, B and F)” (claim 6).
Patent Document 2 suggest in paragraphs [0073] to [0082] that combined hydroxide particles obtained with adjustment made so that the amount of W added was 0.5 mol % based on 100 mol % of a raw material having a molar ratio of Ni:Co:Mn of 0.33:0.33:0.33, and lithium carbonate were mixed such that the Li/Me ratio was about 1.15, and the resulting mixture was fired to produce active material particles including a lithium transition metal composite oxide and having a hollow structure or a solid structure.
Patent Document 3 discloses “an active material having a layered structure and having a composition represented by the following formula (1), wherein the ratio of a full width at half maximum FWHM003 of the (003) plane and a full width at half maximum FWHM104 (104) of the (104) plane in a powder X-ray diffraction diagram is represented by the following formula (2), and the average primary particle size is 0.2 μm to 0.5 μm:LiyNiaCobMncMdOx  (1)[in the formula (1), the element M is at least one element selected from the group consisting of Al, Si, Zr, Ti, Fe, Mg, Nb, Ba and V, and 1.9≤(a+b+c+d+y)≤2.1, 1.0<y≤1.3, 0<a≤0.3, 0<b≤0.25, 0.3≤c≤0.7, 0≤d≤0.1, and 1.9≤x≤2.1.]FWHM003/FWHM104≤0.57  (2)”(claim 1).
Patent Document 3 suggests in examples that citric acid is added to and reacted with an aqueous solution of a raw material mixture of lithium acetate dihydrate, cobalt acetate tetrahydrate, manganese acetate tetrahydrate, nickel acetate tetrahydrate and the like to obtain a precursor, and the precursor is fired to obtain a lithium compound (active material) such as Li1.2Ni0.17Co0.07Mn0.56O2 (see paragraphs [0050], [0051] and [0062]).
Patent Document 4 discloses “an active material for a lithium secondary battery which comprises a solid solution of a sodium-containing lithium transition metal composite oxide having an α-NaFeO2-type crystal structure, wherein the chemical composition formula of the solid solution satisfies Li1+x−yNayCoaNibMncO2+d (0<y≤0.1, 0.4≤c≤0.7, x+a+b+c=1, 0.1≤x≤0.25, −0.2≤d≤0.2), the active material has an X-ray diffraction pattern attributable to a hexagonal crystal (space group P3112), and in the Miller index hkl, the full width at half maximum for the diffraction peak of the (003) is 0.30° or less and the full width at half maximum for the diffraction peak of the (114) plane is 0.50° or less” (claim 1).
In addition, Patent Document 4 suggests in paragraph [0052] that “one of indications of the degree of crystallization is a full width at half maximum for the X-ray diffraction peak as described above; it is necessary that in an X-ray diffraction pattern attributed to the space group P3112, the full width at half maximum for the diffraction peak of the (003) plane be 0.30° or less, and the full width at half maximum for the diffraction peak of the (114) plane be 0.50° or less for improving low-temperature characteristics in the invention; and the full width at half maximum for the diffraction peak of the (003) plane is preferably 0.17° to 0.30°, and the full width at half maximum for the diffraction peak of the (114) plane is preferably 0.35° to 0.50°”.
Patent Document 4 suggests in Examples 1 to 4 that for a lithium transition metal composite oxide obtained by mixing a coprecipitation hydroxide precursor of a transition metal, lithium hydroxide monohydrate and sodium carbonate so as to have various compositions, and firing the resulting mixture at 1000° C., the full width at half maximum for the diffraction peak of the (003) plane is 0.20°, and the full width at half maximum for the diffraction peak of the (114) plane is 0.40° (see paragraphs [0074] to [0078], and Table 1 in paragraph [0102]).
Patent Document 5 discloses “a positive active material for a lithium secondary battery which comprises a lithium transition metal composite oxide represented by the composition formula: Li1+αMe1−αO2 (Me is a transition metal element including Co, Ni and Mn, and 1.2<(1+α)/(1−α)<1.6), wherein in the lithium transition metal composite oxide, the molar ratio (Co/Me) of Co to Me is 0.24 to 0.36, and when the space group R3-m is used as a crystal structure model on the basis of an X-ray diffraction pattern, the full width at half maximum for the diffraction peak attributed to the (003) plane is in a range of 0.204° to 0.303°, or the full width at half maximum for the diffraction peak attributed to the (104) plane is in a range of 0.278° to 0.424°” (claim 1).
In addition, Patent Document 5 suggests in paragraph [0025] that “the peak differential pore volume is preferably 0.85 mm3/(g·nm) or more; and when the peak differential pore volume is 0.85 mm3/(g·nm) or more, a lithium secondary battery excellent in initial efficiency can be obtained”.
In Examples 3 to 6, a lithium transition metal composite oxide prepared from a carbonate precursor, and having a Li/Me ratio of 1.3, a Mn/Me ratio of 0.52 to 0.44 and a Co/Me ratio of 0.28 to 0.36 is disclosed (see paragraphs [0066] to [0068] and Table 1 in paragraph [0096]).
Patent Document 6 discloses “a mixed active material for a lithium secondary battery which comprises two kinds of lithium transition metal composite oxide particles with different particle sizes in which the transition metal oxide has an α-NaFeO2 structure, the transition metal (Me) includes Co, Ni and Mn, and the molar ratio (Li/Me) of lithium (Li) to the transition metal is more than 1, wherein the first lithium transition metal composite oxide particle with a larger particle size has a peak differential pore volume of 0.8 mm3/(g·nm) or more at a pore size in a range of 30 to 40 nm where the differential pore volume determined by a BJH method from an adsorption isotherm using a nitrogen gas adsorption method shows the maximum value, and the second lithium transition metal composite oxide particle with a smaller particle size has a peak differential pore volume of 0.5 mm3/(g·nm) or less at a pore size in a range of 50 to 70 nm where the differential pore volume determined by a BJH method from an adsorption isotherm using a nitrogen gas adsorption method shows the maximum value” (claim 1).
Patent Document 6 suggests in examples that the second lithium transition metal composite oxide particle is prepared by mixing lithium hydroxide monohydrate with a coprecipitation hydroxide precursor, and firing the resulting mixture (see paragraphs [0078] to [0080]), and that “the second lithium transition metal composite oxide particles in Examples 1 to 16 and Comparative Examples 1 to 4 to 6 and 8 had a peak differential pore volume of 0.3 to 0.5 mm3/(g·nm) at a pore size in a range of 50 to 70 nm” (see paragraph [0116]).